Cellular processes are ultimately regulated by proteins and enzymes. The control of the key enzymes of many cellular processes is by allosteric regulation which may be coupled of phosphorylation - a covalent post translation switch that changes protein function. Glycogen metabolism is the most basic process for regulating energy stores of cells and is controlled by glycogen phosphorylase. Using X-ray crystallography, the three dimensional structures of glycogen phosphorylases from muscle, liver, brain and yeast cells will be defined. The structure role of the covalent phosphorylation of these enzymes and its relationship role to the allosteric regulation by small molecule ligands will be determined.